1. Field of the Invention
This invention relates generally to the field of metal alloys useful for metal plating a wide variety of articles, including printed circuit boards and other electronic packaging devices. In particular, the invention relates to additives and their use as a grain refiner in tin and tin alloy plating formulations. Furthermore, the present invention relates to additives which improve the stability of metal plating solutions, particularly copper-based systems.
2. Background
For many years, tin-lead alloy electroplating has been the process of choice in applications requiring attachment of electronic components to printed wiring boards by soldering or reflowing. Such tin-lead alloys have been used as solders and solderable finishes on components. The temperature at which the solder melts is highly important. For tin-lead alloys the temperature of 183xc2x0 C. (eutectic Snxe2x80x94Pb) matches very well the currently used materials for electronic parts. Much higher melting temperatures may result in deformations of the printed wiring board laminate, whereas distinctly reduced liquidus temperatures may lead to unwanted melting of the solder joint during other thermal operations of the assembly process.
Due to its toxic properties lead will likely be banned in electronic parts in the near future. Many tin-lead alternatives have been suggested, including pure tin, tin-silver, tin-copper, and tin-bismuth.
Electroplating articles with tin coatings is generally well known in the industry. Electroplating methods involve passing a current between two electrodes in a plating solution where one electrode is the article to be plated. A common plating solution would be an acid tin plating solution containing (1) a dissolved tin salt (such as tin alkylsulfonate), (2) an acidic electrolyte (such as alkylsulfonic acid) in an amount sufficient to impart conductivity to the bath and (3) additives (such as surfactants, brighteners, levelers and suppressants) to enhance the effectiveness and quality of plating. For example, U.S. Pat. No. 5,174,887 (Federman et al.) discloses a process for the high speed electroplating of tin having as a surfactant an alkylene oxide condensation product of an organic compound having at least one hydroxy group and 20 carbon atoms or less. The organic compounds include an aliphatic hydrocarbon of between 1 and 7 carbon atoms, an unsubstituted aromatic compound or an alkylated aromatic compound having 6 carbon atoms or less in the alkyl moiety.
It is also well known to electroplate articles with tin-alloys. For example, U.S. Pat. No. 6,176,996 (Moon) discloses certain tin-alloy electroplating baths containing a tin salt, a metal salt selected from zinc, cobalt, bismuth or copper salts, methanesulfonic acid, a conductive compound and a complexing agent.
A problem with tin electroplating baths is instability, i.e. sludge may form in the bath with time. Such sludge results from oxidation and subsequent precipitation of the metals in the bath. A number of additives have been proposed to prevent or reduce such sludge formation, however such additives adversely affect certain characteristics of the metal deposit, such as grain refinement.
It thus would be desirable to have new electroplating compositions. It would be particularly desirable to have new electroplating compositions that have increased stability, i.e. prevent or reduce sludge formation. It also would be particularly desirable to have new electroplating compositions which exhibit improved grain refinement.
We have now found new electroplating compositions that effectively plate a wide variety of articles, including printed circuit boards and other electronic packaging devices. Electroplating compositions of the invention exhibit enhanced grain refinement and stability. Using compositions of the invention, precipitation of metals from the plating solutions is eliminated or at least significantly reduced.
Compositions and methods of the invention are particularly useful for depositing solderable finishes on electronic components such as printed wiring boards, semiconductor packaging, lead frames and connectors, and for depositing solder bumps such as those used on integrated circuits. The present compositions also may be used in a wide variety of other electronic device applications, such as in the manufacture of integrated circuits, optoelectronic devices and the like.
Electroplating compositions of the invention are characterized in significant part by an additive that dually functions as a grain refiner and a stabilizer. Preferably, such additive comprises one or more non-aromatic compounds having xcfx80 electrons that can be delocalized. Examples of such compounds include an xcex1,xcex2 unsaturated system or other conjugated system that contains a proximate (e.g., within 1-2 carbons of a carbon-carbon double bond) electron-withdrawing group.
Cyclic systems that contain endocyclic conjugation are generally preferred. Particularly preferred are keto-enole systems. Even more preferred are cyclic keto-enole systems which contain a rigid arrangement of two oxygen donors, particularly where the enole functionality is endocylic.
Keto functionalities having a low pKa value also are particularly preferred systems for use as stabilizer additives in electroplating compositions of the invention.
Preferable concentrations for such present additive for use in electroplating compositions of the invention are between about 2 and about 10,000 mg per L, more preferably between about 50 and about 2000 mg per L, in acid plating solutions.
Preferably, the present additive concentration is maintained at such concentrations throughout the entire or at least substantial portion of a plating cycle. Such maintenance of the stabilizer additive concentrations entails regular addition of the stabilizer additive during a plating cycle as the stabilizer additive component is consumed. Additive concentrations and replenishment rates during a plating cycle can be readily determined by known methods, such as the CPVS method as disclosed in U.S. Pat. Nos. 5,252,196 and 5,223,118, both assigned to the Shipley Company, or by the cyclic voltammetric stripping (CVS) methods, or liquid chromatography.
In addition to such a grain refiner/stabilizer additive, the plating bath may optionally contain one or more reducing agents, grain refiners such as hydroxy aromatic compounds, wetting agents, brightening agents, compounds to extend the current density range, such as carboxylic acids, levelers and the like. Mixtures of additives may also be used in the present invention.
The invention also includes articles of manufacture, including electronic devices such as printed circuit boards, multi-chip modules, lead frames, connectors, semiconductor packaging, optoelectronic devices and the like that contain a deposit produced from a plating solution of the invention.
Other aspects of the invention are discussed infra.
Electroplating compositions of the invention suitably contain a tin salt, an electrolyte preferably an acidic aqueous solution, and a stabilizer additive comprising one or more non-aromatic compounds having xcfx80 electrons that can be delocalized. Compositions of the invention may optionally contain one or more reducing agents, grain refiners such as hydroxy aromatic compounds, wetting agents, brightening agents, compounds to extend the current density range, such as carboxylic acids, levelers and the like.
Compositions and methods of the invention are characterized in large part by the use of one or more stabilizer additives. Electroplating compositions of the invention comprising such stabilizer additives exhibit enhanced grain refinement and improved stability. Using compositions of the invention, precipitation of metals from the plating solutions is eliminated or at least significantly reduced.
Preferred stabilizer additives include those compounds having an xcex1,xcex2 unsaturated system or other conjugated system that contains a proximate (e.g., within 1-2 carbons of a carbon-carbon double bond) electron-withdrawing group.
Cyclic systems that contain endocyclic conjugation are generally preferred. Particularly preferred are keto-enole systems. Even more preferred are cyclic keto-enole systems which contain a rigid arrangement of two oxygen donors, particularly where the enole functionality is endocylic. See, e.g., representative formulae IA-IB below. 
In simple cases (Rxe2x80x3=H, alkyl, OR, and the like), and the equilibrium lies well to the left. The keto form differs from the enole form in possessing a Cxe2x80x94H, a Cxe2x80x94C, and a Cxe2x95x90O bond where the enole has a Cxe2x95x90C, a Cxe2x80x94O, and an Oxe2x80x94H bond. Further aspects of keto-enole tautomerism are presented in Advanced Organic Chemistry, J. March, 4th ed., John Wiley and Sons, 1992, note in particular pp. 70-74 and 585-587.
Particularly preferred keto-enole tautomers are shown below as represented by formulae IIA-IIC. 
Keto functionalities having a low pKa also are particularly preferred systems for use as the present grain refiner/stabilizer additives in electroplating compositions of the invention. This is due to the applied pH in the plating solution, which is acidic for tin plating. Particularly preferred compounds that include this structural feature include hydroxylated gamma-pyrones, e.g., maltol, ethylmaltol, kojic acid, meconic acid, comenic acid, hydroxylated benzoquinones such as chloranilic acid, dihydroxy benzoquinone, hydroxylated naphtoles such as chromotropic acid, several anthraquinones, hydroxylated pyridones, cyclopentandiones, hydroxy-furanones, hydroxy-pyrrolidones, cyclohexanediones, and the like.
We have observed enhanced grain refinement and polarization in the low current density range, particularly when using stabilizer additives such as maltol, ethylmaltol, 3-methyl-1,2-cyclopentanedione and 2,5-dimethyl-4-hydroxy-3-furanone. In addition, the gamma-pyrones appear to solubilize tin(IV) in tin and tin alloy plating baths, thereby preventing or reducing the amount of precipitation of tin(IV). In an alternate embodiment, the gamma-pyrones also stabilize electroless copper solutions against plating-out.
Preferable concentrations for grain refiner/stabilizer additives for use in electroplating compositions of the invention are between about 2 and about 10,000 mg per L, more preferably between about 50 and about 2000 mg per L, in acid plating solutions. However, good results have been achieved with even lower concentrations, e.g. copper electroless plating formulations having a stabilizer additive concentration of at least about 2 mg per L of the metallization solution.
Electroplating compositions and methods of the invention are particularly well suited for use with tin and tin-alloy plating baths including, but not limited to, tin-silver, and tin-copper baths. Electroplating compositions and methods of the invention also are well suited for use with zinc and zinc-alloy plating baths. However, such compositions also are generally applicable to systems containing other alloy materials such as bismuth, indium, antimony, and gold.
Electroplating baths containing the electroplating compositions of the present invention are typically prepared by adding to a vessel one or more acidic electrolytes, followed by one or more tin compounds and optionally one or more other alloy compounds, e.g., copper compound(s), silver compound(s) and the like, and then one or more other additives, including the present grain refiner/stabilizer additive. Other orders of addition of the components of the compositions of the present invention may be used. Once the bath is prepared, undesired material is removed, such as by filtration, and then water is added to adjust the final volume of the bath. The bath may be agitated by any known means, such as stirring, swirling or shaking, for increased plating speed.
As discussed above, electroplating solutions of the invention are particularly useful for depositing solderable finishes on electronic components such as printed wiring boards, optoelectronic devices, semiconductor packaging, microchip module packaging, components, contacts, chip capacitors, chip resistors, lead frames and connectors, and for depositing solder bumps such as those used on integrated circuits.
Any substrate that can be electrolytically plated with a metal is suitable for plating according to the present invention. Suitable substrates include, but are not limited to: copper, copper alloys, nickel, nickel alloys, nickel-iron containing materials, electronic components, plastics, and the like. Suitable plastics include plastic laminates, such as printing wiring boards, particularly copper clad printed wiring boards.
The present grain refiner/stabilizer additives are useful in a wide variety of electrolytic and electroless plating baths, and preferably electrolytic plating baths. Suitable plating baths are those containing one or more metals selected from tin, copper, nickel, silver and the like, and preferably tin.
The one or more tin compounds useful in the present invention are any solution soluble tin compound. Suitable tin compounds include, but are not limited to tin salts, such as tin halides; tin sulfates; tin alkane sulfonate such as tin methane sulfonate, tin ethanesulfonate and tin propanesulfonate; tin aryl sulfonate such as tin phenyl sulfonate and tin toluene sulfonate; tin alkanol sulfonate; and the like. When tin halide is used, it is preferred that the halide is chloride. It is preferred that the tin compound is tin sulfate, tin chloride, tin alkane sulfonate or tin aryl sulfonate, and more preferably tin sulfate or tin methane sulfonate. Suitable tin alkane sulfonates include tin methanesulfonate, tin ethanesulfonate and tin propanesulfonate. Suitable tin aryl sulfonates include tin phenylsulfonate and tin toluenesulfonate. The tin compounds useful in the present invention are generally commercially available from a variety of sources and may be used without further purification. Alternatively, the tin compounds useful in the present invention may be prepared by methods known in the literature.
The amount of tin compound useful in the electrolyte compositions of the present invention is any amount that provides a tin content typically in the range of 2 to 100 g/L, and preferably 10 to 70 g/L. When the compositions of the present invention are used in a low speed plating process, the amount of tin present in the electrolyte composition is typically in the range of 2 to 60 g/L, and preferably 5 to 20 g/L. When the compositions of the present invention are used in a high speed plating process, the amount of tin present in the electrolyte composition is typically in the range of 20 to 100 g/L, and preferably 40 to 70 g/L. When the compositions of the present invention are used in high speed tin plating of steel, the amount of tin is typically in the range of 5 to 50 g/L, and preferably 10 to 30 g/L. Mixtures of tin compounds may also be used advantageously in the present invention, provided that the total amount of tin is in the range of from 2 to 100 g/L.
Any acidic electrolyte that is solution soluble and does not otherwise adversely affect the electrolyte composition may be used advantageously in the present invention. Suitable acidic electrolytes include, but are not limited to alkane sulfonic acids, such as methanesulfonic acid, ethanesulfonic acid and propanesulfonic acid, aryl sulfonic acids such as phenylsulfonic acid or toluenesulfonic acid, sulfuric acid, sulfamic acid, hydrochloric acid, hydrobromic acid and fluoroboric acid. Mixtures of acidic electrolytes are particularly useful, such as, but not limited to, mixtures of alkane sulfonic acids and sulfuric acid. Thus, more than one acidic electrolyte may be used advantageously in the present invention. The acidic electrolytes useful in the present invention are generally commercially available and may be used without further purification. Alternatively, the acidic electrolytes may be prepared by methods known in the literature.
Typically, the amount of acidic electrolyte is in the range of 10 to 400 g/L, and preferably 100 to 250 g/L. When the compositions of the present invention are used in the high speed tin plating of steel, the acidic electrolyte is typically present in an amount in the range of 20 to 80 g/L, and preferably 30 to 60 g/L. It is preferred that when the tin compound is a halide that the acidic electrolyte is the corresponding acid. For example, when tin chloride is used in the present invention, it is preferred that the acidic electrolyte is hydrochloric acid.
One or more alloying metals may be added to the present electroplating baths. Suitable alloying metals for a tin electroplating bath include, but are not limited to, copper, silver, zinc, bismuth, indium, antimony and gold. Preferred alloying metals are copper and silver. Such alloying metals are typically in the form of any solution soluble compound.
Suitable metal compounds include, but are not limited to metal halides, metal acetates, metal fluoroborates, metal sulfate, metal alkane sulfonate, metal alkanol sulfonate, metal nitrates and the like. When copper halide is used, it is preferred that the halide is chloride. It is preferred that the metal compound is metal sulfate, metal alkane sulfonate or mixtures thereof, and more preferably copper sulfate, copper methane sulfonate, silver methane sulfonate or mixtures thereof. Mixtures of alloying metal compounds may be used advantageously in the present invention. Thus, binary, ternary and high metal alloys may be produced. The alloying metal compounds useful in the present invention are generally commercially available or may be prepared by methods known in the literature.
The amount of alloying metal compound useful in the electrolyte compositions of the present invention is any amount that provides a metal content typically in the range of 0.01 to 10 g/L, and preferably 0.1 to 3 g/L. For a tin-copper alloy electroplating bath, the amount of copper present is typically in the range of 0.01 to 5 g/L, and preferably 0.02 to 2 g/L. For a tin-copper alloy electroplating bath used in high speed plating process, the amount of copper is typically in the range of 0.5 to 10 g/L, and preferably 0.5 to 5 g/L.
Reducing agents may be added to the electrolyte composition of the present invention to assist in keeping the tin in a soluble, divalent state. Suitable reducing agents include, but are not limited to, hydroquinone and hydroxylated aromatic compounds, such as resorcinol glycol, and preferably polyethylene glycol. Such polyalkylene glycols are generally commercially available from a variety of sources and may be used without further purification.
Typically, the polyalkylene glycols useful in the present invention are those having an average molecular weight in the range of from about 200 to about 100,000, and preferably from about 900 to about 20,000. Such polyalkylene glycols are present in the electrolyte compositions of the present invention in an amount of from about 0.1 to about 15 g/L, preferably from about 0.25 to about 10 g/L, and more preferably from about 0.5 to about 8 g/L.
Other suitable suppressors include, but are not limited to, amines such as ethoxylated amines, polyoxyalkylene amines and alkanol amines; amides; alkylpolyether sulfonates; complexing surfactants such as alkoxylated diamines; and complexing agents for cupric or cuprous ions which include entprol, citric acid, edetic acid, tartaric acid, potassium sodium tartrate, acetonitrile, cupreine and pyridine. Typically, such other suppressors are used in the same amounts as those described above.
Particularly suitable surfactants for plating compositions of the invention are commercially available polyethylene glycol copolymers, including polyethylene glycol copolymers. Such polymers are available from e.g. BASF (sold by BASF under Tetronic and Pluronic tradenames), and copolymers from Chemax. A butylalcohol-ethylene oxide-propylene oxide copolymer having a MW of about 1800 from Chemax is particularly preferred.
It will be appreciated by those skilled in the art that carboxy aromatic compounds or other wetting agents may be added to the electrolyte compositions of the present invention to provide further grain refinement. Such carboxy aromatic compounds function as grain refiners and may be added to the electrolyte composition of the present invention to further improve deposit appearance and operating current density range. A wide variety of such carboxy aromatic compounds are well known to those skilled in the art, such as picolinic acid, nicotinic acid, and isonicotinic acid. Suitable other wetting agents include, but are not limited to: alkoxylates, such as the polyethoxylated amines JEFFAMINE T-403 or TRITON RW, or sulfated alkyl ethoxylates, such as TRITON QS-15, and gelatin or gelatin derivatives. The amounts of such grain refiners useful in the present invention are well known to those skilled in the art and typically are in the range of 0.01 to 20 g/L, preferably 0.5 to 8 g/L, and more preferably 1 to 5 g/L.
The present metal electroplating compositions are suitably used in a similar manner as prior metal electroplating baths, except that one or more of the present grain refiner/stabilizer additives are employed and preferably maintained at the noted concentration throughout a plating cycle. The electrolyte compositions of the present invention are useful in any plating method where a tin or tin-alloy deposit is desired. Suitable plating methods include, but are not limited to barrel plating, rack plating and high speed plating. A tin or tin-alloy deposit may be plated on a substrate by the steps of contacting the substrate with the electrolyte composition described above and passing a current through the electrolyte to deposit the tin or tin-alloy on the substrate.
The electrolyte compositions of the present invention and plating baths prepared therefrom typically are acidic, i.e. having a pH of less than 7, typically less than 1. If a composition of a specific pH is desired, such as in the range of 2.5 to 4.5, then appropriate adjustment of the pH is necessary. Such pH adjustment may be made by any known methods, such as the addition of base or use of lesser amounts of acidic electrolyte. Such electrolyte compositions of the present invention having a pH of in the range of 2.5 to 4.5 are particularly suited for use in the plating of components containing pH-sensitive materials such as, for example, glass or ceramic capacitors and resistors.
Plating baths of the invention are preferably employed at or above room temperature, e.g. up to and somewhat above 25xc2x0 C. Typical temperatures are in the range of, but not limited to, 60xc2x0 to 150xc2x0 F. (15xc2x0 to 66xc2x0 C.) or higher, and preferably 70xc2x0 to 125xc2x0 F. (21xc2x0 to 52xc2x0 C.), and more preferably 75xc2x0 to 120xc2x0 F. (23xc2x0 to 49xc2x0 C.). It will be appreciated by those skilled in the art that lower plating temperatures are typically used with low speed, or non-high speed, plating systems and/or the use of brighteners. It also will be appreciated by those skilled in the art that higher temperatures are used with high speed plating systems. The plating composition is preferably agitated during use such as by air sparger, work piece agitation, impingement or other suitable method. Plating is preferably conducted at a current ranging from 1 to 2000 ASF (0.1 to 200 ASD) depending upon the metal to be plated, the particular plating method used and the substrate characteristics. When a non-high speed electroplating process is used, the current density is typically in the range of 1 to 40 ASF, and preferably 1 to 30 ASF. For example, when the electrolyte compositions of the present invention are used to deposit tin on connector strips in a high speed plating processes, a suitable current density is 10 to 60 ASD, resulting in a tin deposit having a thickness of typically from 1 to 15 microns. Plating time may range from about 5 minutes to 1 hour or more, depending on the difficulty of the work piece. See generally the examples which follow for exemplary preferred procedures.
The present invention is particularly suitable for depositing a solderable finish on an electronic device substrate. A wide variety of substrates may be plated with the compositions of the invention, as discussed above. Suitable substrates include, but are not limited to printed wiring boards, optoelectronic devices, semiconductor packages, microchip module packages, components, contacts, chip capacitors, chip resistors, lead frames, connectors, or integrated circuits. See the examples which follow for exemplary substrates plated in accordance with the invention.
All documents mentioned herein are fully incorporated herein by reference. The following non-limiting examples are illustrative of the invention.